The Shane et al. U.S. Pat. No. 3,404,061, issued Oct. 1, 1968 discloses a flexible ribbon or tape of expanded (foamed) graphite which neither includes a binder nor a coating. This material, both in flat and transversally corrugated form is available in a variety of widths and thicknesses, e.g. one-quarter to one inch wide and 0.005-0.020 inch from Union Carbide Corporation, under the trademark Grafoil.RTM.. In its least dense form the sheets of the graphite foam have a density somewhat less than 5 pounds per cubic foot. The foamed material can be compressed to a density of about 137 pounds per cubic foot without resorting to extreme measures. The flexible corrugated tape, as presently commercially available has a density which is intermediate this range. A typical use of the flexible graphite tape product, particularly in its corrugated form, is wrapped and compressed in situ stuffing box seals. The bulk density of the corrugated tape when it is wrapped convolutely about a valve stem or other shaft is sufficiently low that a convolute wrap that is approximately equal in thickness to the width of the packing chamber of a stuffing box, can be axially compressed by about 50 percent as the packing gland is tightened to the requisite degree. As the packing ring thus being formed is compressed, the internal space within it and the voids in the foam are correspondingly extinguished, providing a ring the internal and external diameters of which match those of the packing chamber. As the convolute wrapping of tape is crumpled by axial compression as the packing gland is tightened, the individual layers tend to become zig-zagged in transverse cross-section, especially within the bulk of the resulting packing ring. Such a packing ring and method for manufacturing it in situ are disclosed in the Schnitzler U.S. Pat. No. 4,068,853.
Because the preferred material used in making the product of the present invention is the flexible, expanded graphite tape mentioned above, and an important contemplated use of the product of the present invention is stuffing box packing, the entire disclosure of the abovementioned Shane et al and Schnitzler patents are incorporated herein by reference.
Presently, flexible graphite is a preferred material for making high temperature valve stem packing for high pressure steam service, such as in nuclear-powered electric generating plants. There are other, comparable uses for which sealing materials made of flexible graphite are preferred in the trade. So far as the present inventor is aware, until the present invention was made, flexible graphite has been provided to the trade, for sealing usage, in the following forms: as sheets, as ribbons and as die-formed rings.
It is understood that the basic flexible graphite product is foamed sheet of various thicknesses. This basic sheet product is cut into various lengths and widths to provide the sheet product that is sold, which is used as gasket material. The basic sheet product is slit into strips of various widths and preferably run through corrugating rolls to provide the ribbon, strip product that is sold. And the strip is convolutely wound onto mandrels and die-molded to produce rings of various internal diameters, various sizes and proportions of generally rectangular transverse cross sectional shape, and various thicknesses.
Although the stem packing products made of flexible graphite, both in strip form and in pre-formed ring form are well received in the marketplace and have an excellent reputation, there are drawbacks to their use which the present inventor believes have significantly constrained market penetration of this excellent stem packing material.
As to the corrugated flexible graphite ribbon, it is rather difficult and time-consuming to install. If a valve packing is leaking on a steam line and needs to be replaced, generally it is necessary to shut-down that line, disassemble the stuffing box, pull out the worn packing and inspect it for any unusual signs apart from normal wear. Then, the maintenance personnel must determine the amount of corrugated flexible graphite ribbon needed, e.g. using the formula, EQU L=3.7(A-B)C/W
in which:
L=length of ribbon, in feet, PA1 A=stuffing box I.D., in inches, PA1 B=shaft O.D., in inches, PA1 C=stuffing box depth, in inches, and PA1 W=initial width of ribbon, in inches.
From the ribbon stock, a piece is unrolled and cut to the determined length L. Next, this piece is convolute wound around a mandrel the same O.D. as the shaft, or directly on the shaft and slipped into the stuffing box. Normally, a stuffing box is much deeper than the width of the ribbon stock, so it is necessary to axially compress this first wound pile of ribbon, in place, to approximately half its initial thickness, and then to repeat the foregoing steps on successive lengths of corrugated flexible graphite ribbon stock, until the stuffing box is full to the requisite depth, e.g. two-thirds to three-quarters full, whereupon the packing gland, bonnet, other seals, retainers, anti-extrusion ring, etc., are reinstalled and the respective line tested and placed back in service. In many cases, it is not feasible to take the steam line out of service for a long enough time to permit the valve to cool down to a comfortable working temperature, and so the above-described steps often must be carried out while the valve is still very hot. The procedure for changing valve stem packing using corrugated ribbon to make rings in situ is lengthy, stressful and costly. Maintenance personnel tend to praise the performance of the graphite stem packing material in high temperature steam service, but to dread having to perform a stem packing change using the ribbon, as just outlined.
Until now, pre-formed graphite stem packing rings, factory-made from corrugated Grafoil ribbon, has been the most convenient form of the product, but it has also been by far the most expensive form of the product, so far as material cost is concerned. A typical electrical power generating plant (or comparable industrial plant) has hundreds of valves, in an assortment of sizes. While many of the valves may require the same cross-sectional size and shape of packing, the O.D. of the valve stems will vary from valve to valve depending on the diameter of pipeline that the valve serves, and depending upon which manufacturer made the valve. Therefore, a plant that determines to rely on pre-formed rings in order to limit labor and downtime costs necessarily incurs the cost of stocking expensive pre-formed rings in all the sizes they will need.